The use of heating elements to fix toner to print media in electrophotographic printing systems is well known. Prior art technology employs one or more resistive heating elements enclosed in a glass bulb which is inserted into a cylinder formed of a thermally conductive material such as aluminum. The cylinder is coated with a material, such as TEFLON, to reduce toner adhesion to the surface. This embodiment of a fixing device is typically referred to as a fuser. The heat generated by the resistive heating element is transferred to the exterior surface of the fuser through radiation, convection and thermal conduction through the wall of the cylinder. Frequently, the glass bulb is filled with a halogen gas to allow the heating element to be operated at a higher temperature. Another prior art fixing device implementation, known as an instant on fuser, includes a strip of material forming a resistive heating element. The resistive heating element can be formed on the ceramic substrate through a thick film deposition process. The resistive heating element is covered by a coating of glass. The coating of glass permits low friction rotation of a film sleeve over the glass as well as providing electrical insulation. Typically, in an instant on fuser, the resistive heating element is fabricated on the ceramic substrate with the electrical connections at one end of the long axis of the fuser. Multiple resistive heating elements may be used in the instant on fuser.
A significant technical problem encountered in the use of fixing devices is the maintenance of a uniform temperature across the portion of the surface of the fixing device contacting the print media. Generally, a single temperature sensor is located near one end of the surface of the fixing device outside the path the print media follows as it passes over the fixing device. Alternative implementations use a temperature sensor located within the print media path. The temperature sensor is part of a circuit which controls the flow of power to heating elements within the fixing device in an attempt to create a uniform temperature profile across the surface of the fixing device. The thermal loading of the print media on the surface of the fixing device results in a decrease in the surface temperature of the fixing device in those locations on the surface in contact with the print media. Because the temperature sensor provides a measure of the temperature on the surface of the fixing device outside of the print media path in an area which is not thermally loaded, an assumption about the surface temperature offset between this area and an area within the print media path must be made to provide effective control of the fixing device surface temperature profile over the width of the print media. As the width of the print media varies, the value of this temperature offset can change substantially as a result of differences in the thermal loading.
Another alternative implementation uses a thermistor located in the print media path. In this implementation, the circuit will compensate for the thermal loading by the print media path. However, portions of the fixing device located outside of the print media path are not thermally loaded and as a result will be heated above the target temperature. High temperature areas on the fixing device can result in warping of the pressure roller contacting the surface of the fixing device, thereby reducing the life of the fixing device.
In addition to the reliability problems created by non-uniform temperatures, the non-uniformities can result in degraded fixing quality. This occurs from the development of locations across the width of the print media for which the fixing device surface temperature is too high or too low for optimum fusing of the toner. Too low of a fusing temperature can result in toner which is not properly fixed to the print media. Too high of a fusing temperature can result in melted toner adhering to the surface of the fixing device, offsetting the toner from the correct location on the print media.
With fixing devices having multiple heating elements, information about the size of the print media on which printing will be performed is used to control the application of power to the multiple heating elements in the fixing device. In the past, sensors have been included in the print engine to detect the size of the print media on which printing will be performed. These have been placed in the paper path to detect the width of the print media moving through the paper path. Based upon the detected width of the print media, the controller applies power to one or more of the heating elements in an attempt to obtain the desired temperature profile across the length of the fixing device.
Multiple heating elements distributed along the length of a fixing device have been employed in an attempt to provide a uniform surface temperature profile for print media having a variety of widths. The electrical power to each of the heating elements in the fixing device is controlled by a separate control circuit. By controlling the duty cycle of the line power applied to each of the heating elements based upon the print media width detected by the printer, a surface temperature profile with greater uniformity for a given media width can be created. However, part of the difficulty involved in controlling the heating elements is providing data to the controller about the width of the print media on which printing will be performed. For standard sized print media, this information is determined from the tray in which the print media is located. For custom sized print media, sensors in the print media path have been used for detecting the print media width. The use of sensors in the print media path to detect a large variety of print media widths is prohibitively expensive. A need exists for a way in which to determine the width of print media without sensors in the print media path and use this information to control the application of power to the fixing device.